A liquid crystal display device is widely used as a light and thin display device. A liquid crystal display device performs image display by using a nature of liquid crystal that a refractive index of liquid crystal changes as an arrangement state of liquid crystal molecules is changed by application of an electric field.
A liquid crystal material originally has birefringence, and a refractive index of a minor axis direction of a molecule and a refractive index of a major axis direction are represented as no and ne, respectively. When linearly-polarized light enters liquid crystal, a phase difference from the original polarization state, which corresponds to a difference between no and ne, is generated, and elliptically-polarized light is obtained. When a polarizing plate is placed where light passing through liquid crystal goes, only light which corresponds to a transmission axis of the polarizing plate passes through, and becomes linearly-polarized light of which the angle is different from that of the incident light. Since a refractive index of liquid crystal changes when an electric field is applied to liquid crystal and the arrangement is changed, different polarization state can be obtained. By appropriately selecting arrangement of liquid crystal and the angle of a polarizing plate, change of light and dark can be made. For display using an electro-optic effect of liquid crystal, substrate surfaces need orientation treatment, and there are several operation modes possible, depending on the initial orientation.
A TN (twisted nematic) mode is widely used as s a typical mode. In the TN mode, a major axis of a liquid crystal molecule is parallel to a substrate under a condition where no electric field is applied, and molecules are arranged between substrates, and twisted with respect to each other. By applying an electric field, the arrangement state of liquid crystal molecules is changed from a state where a major axis of a molecule is parallel to a substrate plane to a state where the major axis of the molecule is vertical to the substrate plane. However, in a case where the initial orientation of the liquid crystal molecules is parallel to the substrate, molecules located close to the substrate surface are not easily operated even when an electric field is applied since there is strong orientation force due to an interaction between the molecules and the substrate, so that these molecules stay almost in the initial condition. Therefore, birefringence of the liquid crystal remains in that portion, and slight light leakage is generated because of phase difference due to this birefringence, hence it is difficult to obtain an ideal dark state.
As for a contrast which is a ratio of luminance of a light state to luminance of a dark state, as the ratio becomes higher, the image quality can be improved. A contrast is one of the elements which determine display performance.
Thus, as a method for improving a contrast ratio, a liquid crystal display device using a vertical alignment mode instead of the conventional TN mode is proposed (for example, refer to a non-patent document 1).
In a vertical alignment mode, liquid crystal molecules are oriented vertical to a substrate surface of a liquid crystal display device beforehand, therefore, birefringence which occurs in a TN mode does not occur, and transmitted light is blocked by a polarizing plate. In this way, there is little light leakage, and a dark state can be obtained easily. At a time of driving, liquid crystal molecules are inclined from the vertical direction by an electric field, so that a light transmitting property due to birefringence of a liquid crystal material can be obtained.
When an electric field is applied to liquid crystal of vertical alignment, the inclined angle of a molecule to the substrate is determined by the electric field intensity, a liquid crystal material, and characteristics of the orientation mode, and the azimuth of the molecule can be an arbitrary value unless special treatment is conducted.
This arbitrariness of the azimuth of the molecule indicates a possibility of local disorder of orientation order. When an actual display device using liquid crystal of vertical alignment is observed, a display defect caused by this disorder is often found. This display defect is referred to as a schlieren structure, and transmission factor of a pixel is decreased when a schlieren structure appears. Furthermore, the schlieren structure moves with switching of images, which makes a residual image, leading to decrease in display quality of the display.
This defect is prevented by performing a rubbing method which is treatment capable of defining the azimuth of a molecule beforehand.
[non-patent document 1] K. Ohmuro, S. Kataoka, T. Sasaki, and Y. Koike, Society For Information Display '97 Digest of Technical Papers, (1997) 845)